ABSTRACT Hot flushes occur in the majority of menopausal women and in young women after estrogen withdrawal. They are characterized by the activation of heat dissipation effectors, including skin vasodilatation, sweating, and cold-seeking behavior. Despite the millions of individuals who experience these symptoms, the cause remains an enigma, and there is little understanding of the neural circuits for estrogen modulation of body temperature. Our laboratory has long hypothesized that kisspeptin/neurokinin B/dynorphin (KNDy) neurons contribute to the generation of flushes, because of their dramatic changes in the hypothalamus of postmenopausal women. In support of this hypothesis, we recently showed that ablation of KNDy neurons in the rat decreases cutaneous vasodilatation and alters the effects of estrogen on thermoregulation. The goal of the present grant is to elucidate the downstream (preoptic) pathways used by estrogen-responsive KNDy neurons to mediate thermoregulatory vasodilatation. KNDy neurons project to key thermoregulatory structures that regulate heat- dissipation effectors: the median preoptic nucleus (MnPO) and medial preoptic area (MPO). Moreover, both these areas express the primary NKB receptor (NK3R) and activation of NK3R in the MnPO reduces body temperature. We hypothesize that NK3R neurons in the MnPO and MPO integrate information from estrogen-responsive KNDy neurons with warm thermal- signals that trigger heat dissipation effectors. The following specific aims are proposed: 1) To evaluate whether NK3R neurons in the MnPO and MPO of Tacr3-EGFP mice are activated by warm-thermal signals from the environment, are modulated by estrogen and receive inputs from KNDy neurons; 2) To perform electrophysiological recordings of NK3R neurons in the preoptic area of the Tacr3-EGFP mouse to determine if they are warm-sensitive, receive inputs from KNDy neurons and if their thermal sensitivity is altered by estrogen or NK3R signaling; 3) To determine if NK3R neurons in the MnPO and MPO are essential for the estrogen modulation of body temperature; 4) To determine if a homologous projection pathway from KNDy neurons to preoptic NK3R neurons exists in the human. These studies will shed light on the integration of reproductive and thermoregulatory systems and provide clues into the etiology of hot flushes. Understanding the mechanisms of hot flushes is essential for designing targeted therapies.